pg_visibility.c

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/*-------------------------------------------------------------------------
 *
 * pg_visibility.c
 *    display visibility map information and page-level visibility bits
 *
 * Copyright (c) 2016-2026, PostgreSQL Global Development Group
 *
 *    contrib/pg_visibility/pg_visibility.c
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "access/heapam.h"
#include "access/htup_details.h"
#include "access/visibilitymap.h"
#include "access/xloginsert.h"
#include "catalog/pg_type.h"
#include "catalog/storage_xlog.h"
#include "funcapi.h"
#include "miscadmin.h"
#include "storage/bufmgr.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "storage/read_stream.h"
#include "storage/smgr.h"
#include "utils/rel.h"
 
PG_MODULE_MAGIC_EXT(
                    .name = "pg_visibility",
                    .version = PG_VERSION
);
 
typedef struct vbits
{
    BlockNumber next;
    BlockNumber count;
    uint8       bits[FLEXIBLE_ARRAY_MEMBER];
} vbits;
 
typedef struct corrupt_items
{
    BlockNumber next;
    BlockNumber count;
    ItemPointer tids;
} corrupt_items;
 
/* for collect_corrupt_items_read_stream_next_block */
struct collect_corrupt_items_read_stream_private
{
    bool        all_frozen;
    bool        all_visible;
    BlockNumber current_blocknum;
    BlockNumber last_exclusive;
    Relation    rel;
    Buffer      vmbuffer;
};
 
PG_FUNCTION_INFO_V1(pg_visibility_map);
PG_FUNCTION_INFO_V1(pg_visibility_map_rel);
PG_FUNCTION_INFO_V1(pg_visibility);
PG_FUNCTION_INFO_V1(pg_visibility_rel);
PG_FUNCTION_INFO_V1(pg_visibility_map_summary);
PG_FUNCTION_INFO_V1(pg_check_frozen);
PG_FUNCTION_INFO_V1(pg_check_visible);
PG_FUNCTION_INFO_V1(pg_truncate_visibility_map);
 
static TupleDesc pg_visibility_tupdesc(bool include_blkno, bool include_pd);
static vbits *collect_visibility_data(Oid relid, bool include_pd);
static corrupt_items *collect_corrupt_items(Oid relid, bool all_visible,
                                            bool all_frozen);
static void record_corrupt_item(corrupt_items *items, ItemPointer tid);
static bool tuple_all_visible(HeapTuple tup, TransactionId OldestXmin,
                              Buffer buffer);
static void check_relation_relkind(Relation rel);
 
/*
 * Visibility map information for a single block of a relation.
 *
 * Note: the VM code will silently return zeroes for pages past the end
 * of the map, so we allow probes up to MaxBlockNumber regardless of the
 * actual relation size.
 */
Datum
pg_visibility_map(PG_FUNCTION_ARGS)
{
    Oid         relid = PG_GETARG_OID(0);
    int64       blkno = PG_GETARG_INT64(1);
    int32       mapbits;
    Relation    rel;
    Buffer      vmbuffer = InvalidBuffer;
    TupleDesc   tupdesc;
    Datum       values[2];
    bool        nulls[2] = {0};
 
    rel = relation_open(relid, AccessShareLock);
 
    /* Only some relkinds have a visibility map */
    check_relation_relkind(rel);
 
    if (blkno < 0 || blkno > MaxBlockNumber)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("invalid block number")));
 
    tupdesc = pg_visibility_tupdesc(false, false);
 
    mapbits = (int32) visibilitymap_get_status(rel, blkno, &vmbuffer);
    if (vmbuffer != InvalidBuffer)
        ReleaseBuffer(vmbuffer);
    values[0] = BoolGetDatum((mapbits & VISIBILITYMAP_ALL_VISIBLE) != 0);
    values[1] = BoolGetDatum((mapbits & VISIBILITYMAP_ALL_FROZEN) != 0);
 
    relation_close(rel, AccessShareLock);
 
    PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
}
 
/*
 * Visibility map information for a single block of a relation, plus the
 * page-level information for the same block.
 */
Datum
pg_visibility(PG_FUNCTION_ARGS)
{
    Oid         relid = PG_GETARG_OID(0);
    int64       blkno = PG_GETARG_INT64(1);
    int32       mapbits;
    Relation    rel;
    Buffer      vmbuffer = InvalidBuffer;
    Buffer      buffer;
    Page        page;
    TupleDesc   tupdesc;
    Datum       values[3];
    bool        nulls[3] = {0};
 
    rel = relation_open(relid, AccessShareLock);
 
    /* Only some relkinds have a visibility map */
    check_relation_relkind(rel);
 
    if (blkno < 0 || blkno > MaxBlockNumber)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("invalid block number")));
 
    tupdesc = pg_visibility_tupdesc(false, true);
 
    mapbits = (int32) visibilitymap_get_status(rel, blkno, &vmbuffer);
    if (vmbuffer != InvalidBuffer)
        ReleaseBuffer(vmbuffer);
    values[0] = BoolGetDatum((mapbits & VISIBILITYMAP_ALL_VISIBLE) != 0);
    values[1] = BoolGetDatum((mapbits & VISIBILITYMAP_ALL_FROZEN) != 0);
 
    /* Here we have to explicitly check rel size ... */
    if (blkno < RelationGetNumberOfBlocks(rel))
    {
        buffer = ReadBuffer(rel, blkno);
        LockBuffer(buffer, BUFFER_LOCK_SHARE);
 
        page = BufferGetPage(buffer);
        values[2] = BoolGetDatum(PageIsAllVisible(page));
 
        UnlockReleaseBuffer(buffer);
    }
    else
    {
        /* As with the vismap, silently return 0 for pages past EOF */
        values[2] = BoolGetDatum(false);
    }
 
    relation_close(rel, AccessShareLock);
 
    PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
}
 
/*
 * Visibility map information for every block in a relation.
 */
Datum
pg_visibility_map_rel(PG_FUNCTION_ARGS)
{
    FuncCallContext *funcctx;
    vbits      *info;
 
    if (SRF_IS_FIRSTCALL())
    {
        Oid         relid = PG_GETARG_OID(0);
        MemoryContext oldcontext;
 
        funcctx = SRF_FIRSTCALL_INIT();
        oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
        funcctx->tuple_desc = pg_visibility_tupdesc(true, false);
        /* collect_visibility_data will verify the relkind */
        funcctx->user_fctx = collect_visibility_data(relid, false);
        MemoryContextSwitchTo(oldcontext);
    }
 
    funcctx = SRF_PERCALL_SETUP();
    info = (vbits *) funcctx->user_fctx;
 
    if (info->next < info->count)
    {
        Datum       values[3];
        bool        nulls[3] = {0};
        HeapTuple   tuple;
 
        values[0] = Int64GetDatum(info->next);
        values[1] = BoolGetDatum((info->bits[info->next] & (1 << 0)) != 0);
        values[2] = BoolGetDatum((info->bits[info->next] & (1 << 1)) != 0);
        info->next++;
 
        tuple = heap_form_tuple(funcctx->tuple_desc, values, nulls);
        SRF_RETURN_NEXT(funcctx, HeapTupleGetDatum(tuple));
    }
 
    SRF_RETURN_DONE(funcctx);
}
 
/*
 * Visibility map information for every block in a relation, plus the page
 * level information for each block.
 */
Datum
pg_visibility_rel(PG_FUNCTION_ARGS)
{
    FuncCallContext *funcctx;
    vbits      *info;
 
    if (SRF_IS_FIRSTCALL())
    {
        Oid         relid = PG_GETARG_OID(0);
        MemoryContext oldcontext;
 
        funcctx = SRF_FIRSTCALL_INIT();
        oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
        funcctx->tuple_desc = pg_visibility_tupdesc(true, true);
        /* collect_visibility_data will verify the relkind */
        funcctx->user_fctx = collect_visibility_data(relid, true);
        MemoryContextSwitchTo(oldcontext);
    }
 
    funcctx = SRF_PERCALL_SETUP();
    info = (vbits *) funcctx->user_fctx;
 
    if (info->next < info->count)
    {
        Datum       values[4];
        bool        nulls[4] = {0};
        HeapTuple   tuple;
 
        values[0] = Int64GetDatum(info->next);
        values[1] = BoolGetDatum((info->bits[info->next] & (1 << 0)) != 0);
        values[2] = BoolGetDatum((info->bits[info->next] & (1 << 1)) != 0);
        values[3] = BoolGetDatum((info->bits[info->next] & (1 << 2)) != 0);
        info->next++;
 
        tuple = heap_form_tuple(funcctx->tuple_desc, values, nulls);
        SRF_RETURN_NEXT(funcctx, HeapTupleGetDatum(tuple));
    }
 
    SRF_RETURN_DONE(funcctx);
}
 
/*
 * Count the number of all-visible and all-frozen pages in the visibility
 * map for a particular relation.
 */
Datum
pg_visibility_map_summary(PG_FUNCTION_ARGS)
{
    Oid         relid = PG_GETARG_OID(0);
    Relation    rel;
    BlockNumber all_visible = 0;
    BlockNumber all_frozen = 0;
    TupleDesc   tupdesc;
    Datum       values[2];
    bool        nulls[2] = {0};
 
    rel = relation_open(relid, AccessShareLock);
 
    /* Only some relkinds have a visibility map */
    check_relation_relkind(rel);
 
    visibilitymap_count(rel, &all_visible, &all_frozen);
 
    relation_close(rel, AccessShareLock);
 
    if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
        elog(ERROR, "return type must be a row type");
 
    values[0] = Int64GetDatum((int64) all_visible);
    values[1] = Int64GetDatum((int64) all_frozen);
 
    PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
}
 
/*
 * Return the TIDs of non-frozen tuples present in pages marked all-frozen
 * in the visibility map.  We hope no one will ever find any, but there could
 * be bugs, database corruption, etc.
 */
Datum
pg_check_frozen(PG_FUNCTION_ARGS)
{
    FuncCallContext *funcctx;
    corrupt_items *items;
 
    if (SRF_IS_FIRSTCALL())
    {
        Oid         relid = PG_GETARG_OID(0);
        MemoryContext oldcontext;
 
        funcctx = SRF_FIRSTCALL_INIT();
        oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
        /* collect_corrupt_items will verify the relkind */
        funcctx->user_fctx = collect_corrupt_items(relid, false, true);
        MemoryContextSwitchTo(oldcontext);
    }
 
    funcctx = SRF_PERCALL_SETUP();
    items = (corrupt_items *) funcctx->user_fctx;
 
    if (items->next < items->count)
        SRF_RETURN_NEXT(funcctx, PointerGetDatum(&items->tids[items->next++]));
 
    SRF_RETURN_DONE(funcctx);
}
 
/*
 * Return the TIDs of not-all-visible tuples in pages marked all-visible
 * in the visibility map.  We hope no one will ever find any, but there could
 * be bugs, database corruption, etc.
 */
Datum
pg_check_visible(PG_FUNCTION_ARGS)
{
    FuncCallContext *funcctx;
    corrupt_items *items;
 
    if (SRF_IS_FIRSTCALL())
    {
        Oid         relid = PG_GETARG_OID(0);
        MemoryContext oldcontext;
 
        funcctx = SRF_FIRSTCALL_INIT();
        oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
        /* collect_corrupt_items will verify the relkind */
        funcctx->user_fctx = collect_corrupt_items(relid, true, false);
        MemoryContextSwitchTo(oldcontext);
    }
 
    funcctx = SRF_PERCALL_SETUP();
    items = (corrupt_items *) funcctx->user_fctx;
 
    if (items->next < items->count)
        SRF_RETURN_NEXT(funcctx, PointerGetDatum(&items->tids[items->next++]));
 
    SRF_RETURN_DONE(funcctx);
}
 
/*
 * Remove the visibility map fork for a relation.  If there turn out to be
 * any bugs in the visibility map code that require rebuilding the VM, this
 * provides users with a way to do it that is cleaner than shutting down the
 * server and removing files by hand.
 *
 * This is a cut-down version of RelationTruncate.
 */
Datum
pg_truncate_visibility_map(PG_FUNCTION_ARGS)
{
    Oid         relid = PG_GETARG_OID(0);
    Relation    rel;
    ForkNumber  fork;
    BlockNumber block;
    BlockNumber old_block;
 
    rel = relation_open(relid, AccessExclusiveLock);
 
    /* Only some relkinds have a visibility map */
    check_relation_relkind(rel);
 
    /* Forcibly reset cached file size */
    RelationGetSmgr(rel)->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM] = InvalidBlockNumber;
 
    /* Compute new and old size before entering critical section. */
    fork = VISIBILITYMAP_FORKNUM;
    block = visibilitymap_prepare_truncate(rel, 0);
    old_block = BlockNumberIsValid(block) ? smgrnblocks(RelationGetSmgr(rel), fork) : 0;
 
    /*
     * WAL-logging, buffer dropping, file truncation must be atomic and all on
     * one side of a checkpoint.  See RelationTruncate() for discussion.
     */
    Assert((MyProc->delayChkptFlags & (DELAY_CHKPT_START | DELAY_CHKPT_COMPLETE)) == 0);
    MyProc->delayChkptFlags |= DELAY_CHKPT_START | DELAY_CHKPT_COMPLETE;
    START_CRIT_SECTION();
 
    if (RelationNeedsWAL(rel))
    {
        XLogRecPtr  lsn;
        xl_smgr_truncate xlrec;
 
        xlrec.blkno = 0;
        xlrec.rlocator = rel->rd_locator;
        xlrec.flags = SMGR_TRUNCATE_VM;
 
        XLogBeginInsert();
        XLogRegisterData(&xlrec, sizeof(xlrec));
 
        lsn = XLogInsert(RM_SMGR_ID,
                         XLOG_SMGR_TRUNCATE | XLR_SPECIAL_REL_UPDATE);
        XLogFlush(lsn);
    }
 
    if (BlockNumberIsValid(block))
        smgrtruncate(RelationGetSmgr(rel), &fork, 1, &old_block, &block);
 
    END_CRIT_SECTION();
    MyProc->delayChkptFlags &= ~(DELAY_CHKPT_START | DELAY_CHKPT_COMPLETE);
 
    /*
     * Release the lock right away, not at commit time.
     *
     * It would be a problem to release the lock prior to commit if this
     * truncate operation sends any transactional invalidation messages. Other
     * backends would potentially be able to lock the relation without
     * processing them in the window of time between when we release the lock
     * here and when we sent the messages at our eventual commit.  However,
     * we're currently only sending a non-transactional smgr invalidation,
     * which will have been posted to shared memory immediately from within
     * smgr_truncate.  Therefore, there should be no race here.
     *
     * The reason why it's desirable to release the lock early here is because
     * of the possibility that someone will need to use this to blow away many
     * visibility map forks at once.  If we can't release the lock until
     * commit time, the transaction doing this will accumulate
     * AccessExclusiveLocks on all of those relations at the same time, which
     * is undesirable. However, if this turns out to be unsafe we may have no
     * choice...
     */
    relation_close(rel, AccessExclusiveLock);
 
    /* Nothing to return. */
    PG_RETURN_VOID();
}
 
/*
 * Helper function to construct whichever TupleDesc we need for a particular
 * call.
 */
static TupleDesc
pg_visibility_tupdesc(bool include_blkno, bool include_pd)
{
    TupleDesc   tupdesc;
    AttrNumber  maxattr = 2;
    AttrNumber  a = 0;
 
    if (include_blkno)
        ++maxattr;
    if (include_pd)
        ++maxattr;
    tupdesc = CreateTemplateTupleDesc(maxattr);
    if (include_blkno)
        TupleDescInitEntry(tupdesc, ++a, "blkno", INT8OID, -1, 0);
    TupleDescInitEntry(tupdesc, ++a, "all_visible", BOOLOID, -1, 0);
    TupleDescInitEntry(tupdesc, ++a, "all_frozen", BOOLOID, -1, 0);
    if (include_pd)
        TupleDescInitEntry(tupdesc, ++a, "pd_all_visible", BOOLOID, -1, 0);
    Assert(a == maxattr);
 
    TupleDescFinalize(tupdesc);
 
    return BlessTupleDesc(tupdesc);
}
 
/*
 * Collect visibility data about a relation.
 *
 * Checks relkind of relid and will throw an error if the relation does not
 * have a VM.
 */
static vbits *
collect_visibility_data(Oid relid, bool include_pd)
{
    Relation    rel;
    BlockNumber nblocks;
    vbits      *info;
    BlockNumber blkno;
    Buffer      vmbuffer = InvalidBuffer;
    BufferAccessStrategy bstrategy = GetAccessStrategy(BAS_BULKREAD);
    BlockRangeReadStreamPrivate p;
    ReadStream *stream = NULL;
 
    rel = relation_open(relid, AccessShareLock);
 
    /* Only some relkinds have a visibility map */
    check_relation_relkind(rel);
 
    nblocks = RelationGetNumberOfBlocks(rel);
    info = palloc0(offsetof(vbits, bits) + nblocks);
    info->next = 0;
    info->count = nblocks;
 
    /* Create a stream if reading main fork. */
    if (include_pd)
    {
        p.current_blocknum = 0;
        p.last_exclusive = nblocks;
 
        /*
         * It is safe to use batchmode as block_range_read_stream_cb takes no
         * locks.
         */
        stream = read_stream_begin_relation(READ_STREAM_FULL |
                                            READ_STREAM_USE_BATCHING,
                                            bstrategy,
                                            rel,
                                            MAIN_FORKNUM,
                                            block_range_read_stream_cb,
                                            &p,
                                            0);
    }
 
    for (blkno = 0; blkno < nblocks; ++blkno)
    {
        int32       mapbits;
 
        /* Make sure we are interruptible. */
        CHECK_FOR_INTERRUPTS();
 
        /* Get map info. */
        mapbits = (int32) visibilitymap_get_status(rel, blkno, &vmbuffer);
        if ((mapbits & VISIBILITYMAP_ALL_VISIBLE) != 0)
            info->bits[blkno] |= (1 << 0);
        if ((mapbits & VISIBILITYMAP_ALL_FROZEN) != 0)
            info->bits[blkno] |= (1 << 1);
 
        /*
         * Page-level data requires reading every block, so only get it if the
         * caller needs it.  Use a buffer access strategy, too, to prevent
         * cache-trashing.
         */
        if (include_pd)
        {
            Buffer      buffer;
            Page        page;
 
            buffer = read_stream_next_buffer(stream, NULL);
            LockBuffer(buffer, BUFFER_LOCK_SHARE);
 
            page = BufferGetPage(buffer);
            if (PageIsAllVisible(page))
                info->bits[blkno] |= (1 << 2);
 
            UnlockReleaseBuffer(buffer);
        }
    }
 
    if (include_pd)
    {
        Assert(read_stream_next_buffer(stream, NULL) == InvalidBuffer);
        read_stream_end(stream);
    }
 
    /* Clean up. */
    if (vmbuffer != InvalidBuffer)
        ReleaseBuffer(vmbuffer);
    relation_close(rel, AccessShareLock);
 
    return info;
}
 
/*
 * The "strict" version of GetOldestNonRemovableTransactionId().  The
 * pg_visibility check can tolerate false positives (don't report some of the
 * errors), but can't tolerate false negatives (report false errors). Normally,
 * horizons move forwards, but there are cases when it could move backward
 * (see comment for ComputeXidHorizons()).
 *
 * This is why we have to implement our own function for xid horizon, which
 * would be guaranteed to be newer or equal to any xid horizon computed before.
 * We have to do the following to achieve this.
 *
 * 1. Ignore processes xmin's, because they consider connection to other
 *    databases that were ignored before.
 * 2. Ignore KnownAssignedXids, as they are not database-aware. Although we
 *    now perform minimal checking on a standby by always using nextXid, this
 *    approach is better than nothing and will at least catch extremely broken
 *    cases where a xid is in the future.
 * 3. Ignore walsender xmin, because it could go backward if some replication
 *    connections don't use replication slots.
 *
 * While it might seem like we could use KnownAssignedXids for shared
 * catalogs, since shared catalogs rely on a global horizon rather than a
 * database-specific one - there are potential edge cases.  For example, a
 * transaction may crash on the primary without writing a commit/abort record.
 * This would lead to a situation where it appears to still be running on the
 * standby, even though it has already ended on the primary.  For this reason,
 * it's safer to ignore KnownAssignedXids, even for shared catalogs.
 *
 * As a result, we're using only currently running xids to compute the horizon.
 * Surely these would significantly sacrifice accuracy.  But we have to do so
 * to avoid reporting false errors.
 */
static TransactionId
GetStrictOldestNonRemovableTransactionId(Relation rel)
{
    RunningTransactions runningTransactions;
 
    if (RecoveryInProgress())
    {
        TransactionId result;
 
        /* As we ignore KnownAssignedXids on standby, just pick nextXid */
        LWLockAcquire(XidGenLock, LW_SHARED);
        result = XidFromFullTransactionId(TransamVariables->nextXid);
        LWLockRelease(XidGenLock);
        return result;
    }
    else if (rel == NULL || rel->rd_rel->relisshared)
    {
        /* Shared relation: take into account all running xids */
        runningTransactions = GetRunningTransactionData();
        LWLockRelease(ProcArrayLock);
        LWLockRelease(XidGenLock);
        return runningTransactions->oldestRunningXid;
    }
    else if (!RELATION_IS_LOCAL(rel))
    {
        /*
         * Normal relation: take into account xids running within the current
         * database
         */
        runningTransactions = GetRunningTransactionData();
        LWLockRelease(ProcArrayLock);
        LWLockRelease(XidGenLock);
        return runningTransactions->oldestDatabaseRunningXid;
    }
    else
    {
        /*
         * For temporary relations, ComputeXidHorizons() uses only
         * TransamVariables->latestCompletedXid and MyProc->xid.  These two
         * shouldn't go backwards.  So we're fine with this horizon.
         */
        return GetOldestNonRemovableTransactionId(rel);
    }
}
 
/*
 * Callback function to get next block for read stream object used in
 * collect_corrupt_items() function.
 */
static BlockNumber
collect_corrupt_items_read_stream_next_block(ReadStream *stream,
                                             void *callback_private_data,
                                             void *per_buffer_data)
{
    struct collect_corrupt_items_read_stream_private *p = callback_private_data;
 
    for (; p->current_blocknum < p->last_exclusive; p->current_blocknum++)
    {
        bool        check_frozen = false;
        bool        check_visible = false;
 
        /* Make sure we are interruptible. */
        CHECK_FOR_INTERRUPTS();
 
        if (p->all_frozen && VM_ALL_FROZEN(p->rel, p->current_blocknum, &p->vmbuffer))
            check_frozen = true;
        if (p->all_visible && VM_ALL_VISIBLE(p->rel, p->current_blocknum, &p->vmbuffer))
            check_visible = true;
        if (!check_visible && !check_frozen)
            continue;
 
        return p->current_blocknum++;
    }
 
    return InvalidBlockNumber;
}
 
/*
 * Returns a list of items whose visibility map information does not match
 * the status of the tuples on the page.
 *
 * If all_visible is passed as true, this will include all items which are
 * on pages marked as all-visible in the visibility map but which do not
 * seem to in fact be all-visible.
 *
 * If all_frozen is passed as true, this will include all items which are
 * on pages marked as all-frozen but which do not seem to in fact be frozen.
 *
 * Checks relkind of relid and will throw an error if the relation does not
 * have a VM.
 */
static corrupt_items *
collect_corrupt_items(Oid relid, bool all_visible, bool all_frozen)
{
    Relation    rel;
    corrupt_items *items;
    Buffer      vmbuffer = InvalidBuffer;
    BufferAccessStrategy bstrategy = GetAccessStrategy(BAS_BULKREAD);
    TransactionId OldestXmin = InvalidTransactionId;
    struct collect_corrupt_items_read_stream_private p;
    ReadStream *stream;
    Buffer      buffer;
 
    rel = relation_open(relid, AccessShareLock);
 
    /* Only some relkinds have a visibility map */
    check_relation_relkind(rel);
 
    if (all_visible)
        OldestXmin = GetStrictOldestNonRemovableTransactionId(rel);
 
    /*
     * Guess an initial array size. We don't expect many corrupted tuples, so
     * start with a small array.  This function uses the "next" field to track
     * the next offset where we can store an item (which is the same thing as
     * the number of items found so far) and the "count" field to track the
     * number of entries allocated.  We'll repurpose these fields before
     * returning.
     */
    items = palloc0_object(corrupt_items);
    items->next = 0;
    items->count = 64;
    items->tids = palloc(items->count * sizeof(ItemPointerData));
 
    p.current_blocknum = 0;
    p.last_exclusive = RelationGetNumberOfBlocks(rel);
    p.rel = rel;
    p.vmbuffer = InvalidBuffer;
    p.all_frozen = all_frozen;
    p.all_visible = all_visible;
    stream = read_stream_begin_relation(READ_STREAM_FULL,
                                        bstrategy,
                                        rel,
                                        MAIN_FORKNUM,
                                        collect_corrupt_items_read_stream_next_block,
                                        &p,
                                        0);
 
    /* Loop over every block in the relation. */
    while ((buffer = read_stream_next_buffer(stream, NULL)) != InvalidBuffer)
    {
        bool        check_frozen = all_frozen;
        bool        check_visible = all_visible;
        Page        page;
        OffsetNumber offnum,
                    maxoff;
        BlockNumber blkno;
 
        /* Make sure we are interruptible. */
        CHECK_FOR_INTERRUPTS();
 
        LockBuffer(buffer, BUFFER_LOCK_SHARE);
 
        page = BufferGetPage(buffer);
        maxoff = PageGetMaxOffsetNumber(page);
        blkno = BufferGetBlockNumber(buffer);
 
        /*
         * The visibility map bits might have changed while we were acquiring
         * the page lock.  Recheck to avoid returning spurious results.
         */
        if (check_frozen && !VM_ALL_FROZEN(rel, blkno, &vmbuffer))
            check_frozen = false;
        if (check_visible && !VM_ALL_VISIBLE(rel, blkno, &vmbuffer))
            check_visible = false;
        if (!check_visible && !check_frozen)
        {
            UnlockReleaseBuffer(buffer);
            continue;
        }
 
        /* Iterate over each tuple on the page. */
        for (offnum = FirstOffsetNumber;
             offnum <= maxoff;
             offnum = OffsetNumberNext(offnum))
        {
            HeapTupleData tuple;
            ItemId      itemid;
 
            itemid = PageGetItemId(page, offnum);
 
            /* Unused or redirect line pointers are of no interest. */
            if (!ItemIdIsUsed(itemid) || ItemIdIsRedirected(itemid))
                continue;
 
            /* Dead line pointers are neither all-visible nor frozen. */
            if (ItemIdIsDead(itemid))
            {
                ItemPointerSet(&(tuple.t_self), blkno, offnum);
                record_corrupt_item(items, &tuple.t_self);
                continue;
            }
 
            /* Initialize a HeapTupleData structure for checks below. */
            ItemPointerSet(&(tuple.t_self), blkno, offnum);
            tuple.t_data = (HeapTupleHeader) PageGetItem(page, itemid);
            tuple.t_len = ItemIdGetLength(itemid);
            tuple.t_tableOid = relid;
 
            /*
             * If we're checking whether the page is all-visible, we expect
             * the tuple to be all-visible.
             */
            if (check_visible &&
                !tuple_all_visible(&tuple, OldestXmin, buffer))
            {
                TransactionId RecomputedOldestXmin;
 
                /*
                 * Time has passed since we computed OldestXmin, so it's
                 * possible that this tuple is all-visible in reality even
                 * though it doesn't appear so based on our
                 * previously-computed value.  Let's compute a new value so we
                 * can be certain whether there is a problem.
                 *
                 * From a concurrency point of view, it sort of sucks to
                 * retake ProcArrayLock here while we're holding the buffer
                 * locked in shared mode, but it should be safe against
                 * deadlocks, because surely
                 * GetStrictOldestNonRemovableTransactionId() should never
                 * take a buffer lock. And this shouldn't happen often, so
                 * it's worth being careful so as to avoid false positives.
                 */
                RecomputedOldestXmin = GetStrictOldestNonRemovableTransactionId(rel);
 
                if (!TransactionIdPrecedes(OldestXmin, RecomputedOldestXmin))
                    record_corrupt_item(items, &tuple.t_self);
                else
                {
                    OldestXmin = RecomputedOldestXmin;
                    if (!tuple_all_visible(&tuple, OldestXmin, buffer))
                        record_corrupt_item(items, &tuple.t_self);
                }
            }
 
            /*
             * If we're checking whether the page is all-frozen, we expect the
             * tuple to be in a state where it will never need freezing.
             */
            if (check_frozen)
            {
                if (heap_tuple_needs_eventual_freeze(tuple.t_data))
                    record_corrupt_item(items, &tuple.t_self);
            }
        }
 
        UnlockReleaseBuffer(buffer);
    }
    read_stream_end(stream);
 
    /* Clean up. */
    if (vmbuffer != InvalidBuffer)
        ReleaseBuffer(vmbuffer);
    if (p.vmbuffer != InvalidBuffer)
        ReleaseBuffer(p.vmbuffer);
    relation_close(rel, AccessShareLock);
 
    /*
     * Before returning, repurpose the fields to match caller's expectations.
     * next is now the next item that should be read (rather than written) and
     * count is now the number of items we wrote (rather than the number we
     * allocated).
     */
    items->count = items->next;
    items->next = 0;
 
    return items;
}
 
/*
 * Remember one corrupt item.
 */
static void
record_corrupt_item(corrupt_items *items, ItemPointer tid)
{
    /* enlarge output array if needed. */
    if (items->next >= items->count)
    {
        items->count *= 2;
        items->tids = repalloc(items->tids,
                               items->count * sizeof(ItemPointerData));
    }
    /* and add the new item */
    items->tids[items->next++] = *tid;
}
 
/*
 * Check whether a tuple is all-visible relative to a given OldestXmin value.
 * The buffer should contain the tuple and should be locked and pinned.
 */
static bool
tuple_all_visible(HeapTuple tup, TransactionId OldestXmin, Buffer buffer)
{
    HTSV_Result state;
    TransactionId xmin;
 
    state = HeapTupleSatisfiesVacuum(tup, OldestXmin, buffer);
    if (state != HEAPTUPLE_LIVE)
        return false;           /* all-visible implies live */
 
    /*
     * Neither lazy_scan_heap nor heap_page_is_all_visible will mark a page
     * all-visible unless every tuple is hinted committed. However, those hint
     * bits could be lost after a crash, so we can't be certain that they'll
     * be set here.  So just check the xmin.
     */
 
    xmin = HeapTupleHeaderGetXmin(tup->t_data);
    if (!TransactionIdPrecedes(xmin, OldestXmin))
        return false;           /* xmin not old enough for all to see */
 
    return true;
}
 
/*
 * check_relation_relkind - convenience routine to check that relation
 * is of the relkind supported by the callers
 */
static void
check_relation_relkind(Relation rel)
{
    if (!RELKIND_HAS_TABLE_AM(rel->rd_rel->relkind))
        ereport(ERROR,
                (errcode(ERRCODE_WRONG_OBJECT_TYPE),
                 errmsg("relation \"%s\" is of wrong relation kind",
                        RelationGetRelationName(rel)),
                 errdetail_relkind_not_supported(rel->rd_rel->relkind)));
}